1. Field of the Invention
The present invention relates to a porous film manufacturing apparatus and, more particularly, to an apparatus for manufacturing a porous film having vapor permeability or both vapor permeability and air permeability and suitable for a material such as a sanitary material for a disposable paper diaper as a typical example, a medical material, or a clothing material.
2. Description of the Related Art
A method disclosed in Published Unexamined Japanese Patent Application No. 1-266150 is known as a conventional method of manufacturing a porous film. In this method, after a large amount of fine inorganic powder is added to a thermoplastic resin, e.g., polyethylene (normally, the amount of powder is 50 vol % or more of the resin), the compound is subsequently formed into a film, and the film is uniaxially or biaxially stretched at a high magnification to form open cells in the interface between the resin and the inorganic powder, thus forming fine pores communicating with each other in the form of a maze.
In the conventional manufacturing method described above, however, the following problems are posed.
(1) Since a large amount of inorganic powder is added, the essential properties (e.g., strength, softness and transparency) of a resin constituting a film are considerably deteriorated, and a plastic-like film cannot be substantially obtained. PA1 (2) Since a large amount of inorganic powder is added, and the resultant film is uniaxially or biaxially stretched at a high magnification, this method cannot be applied to a film having elasticity, such as an elastomer film. PA1 (3) Since fine pores on the submicron order are formed in the resultant film so as to communicate with each other in the form of a maze, the film has almost no air permeability although it has vapor permeability. For this reason, practical applications of the film are undesirably limited. PA1 feed means for feeding an elongated film; PA1 a perforating unit including a first rotatable roll having a surface on which a large number of particles, each having acute corner portions and a Mohs hardness value of not less than 5, are deposited, and a second roll which is rotatable in a direction reverse to a rotating direction of the first roll, the first and second rolls being arranged to oppose each other and to cause the elongated film to pass therebetween, one of the rolls being stationary and the other roll being movable in a direction to oppose the one roll; and PA1 pressure control means, arranged near two end portions of the movable roll of the unit, for controlling a pressure applied from each of the rolls to the film. PA1 (1) Since some portions of an elongated film do not receive a sufficiently high pressure because of the displacement of the first roll, the corresponding portions are not perforated. This makes it difficult to form uniform through pores and the like. PA1 (2) Owing to the displacement of the first roll, the first and second rolls are intermittently rotated and hence cannot be smoothly rotated. As a result, wrinkles are left on the entire surface of the film. PA1 (3) If the thickness of the film varies, it may be cut while it passes between the rolls. As a result, it is difficult to continuously form through pores or blind pores in the elongated film. PA1 (4) If an elongated film made of a metal is used, biting is caused between the rolls to stop rotation of the rolls. As a result, through pores or the like cannot be formed. PA1 (1) Since a pressure applied from the rolls to an elongated film upon passage of the film between the first and second rollers which are rotated in the opposite directions can be made uniform along the direction of width of the film, through pores and blind pores can be uniformly formed in the film at a high density. In addition, since the perforating operation is performed by mechanical force using the large number of particles, each having acute corner portions and a Mohs hardness value of 5 or more, which are deposited on the surface of the first roll, a large number of uniform through pores and blind pores, each having a small size selected in the range of sub-.mu.m to several tens .mu.m, can be formed in the elongated film without almost any deterioration in essential properties of the film material. PA1 (2) Since the first and second rolls can be smoothly rotated, perforation can be performed without leaving wrinkles on the elongated film. In addition, since each roll can be rotated at high speed, the perforation process performance with respect to the film can be greatly improved. PA1 (3) Even if the thickness of the elongated film varies, it is not cut when it passes between the rolls, and through pores or blind pores can be continuously formed in the elongated film. PA1 (4) Even if a metal elongated film is used, since biting between the rolls can be prevented, the film can be continuously perforated. The metal porous film manufactured by such a perforation process has excellent vapor and air permeability, and the glossiness of the surface of the film is improved. PA1 feed means for feeding an elongated film; PA1 a perforating unit including a first rotatable roll having a surface on which a large number of particles, each having acute corner portions and a Mohs hardness value of not less than 5, are deposited, a second roll which has a surface consisting of a hard material and is rotatable in a direction reverse to a rotating direction of the first roll, and a third roll which has a surface consisting of a soft material and is rotatable in a direction reverse to the rotating direction of the first roll, the first to third rolls being arranged to oppose each other with the first roll being a middle roll of the first to third rolls and to cause the elongated film to pass between the first and second rolls and between the first and third rolls, the first roll being stationary and the second and third rolls being movable in a direction to oppose the first roll; PA1 first pressure control means, arranged near two end portions of the second roll of the unit, for controlling pressures applied from the first and second rolls to the film; and PA1 second pressure control means, arranged near two end portions of the third roll of the unit, for controlling pressures applied from the first and third rolls to the film.
As other porous film manufacturing methods, mechanical perforation methods such as a needle punch method and a heat fusion perforation method are known. In the needle punch method, a thermoplastic resin film is perforated by pressing heated needles against the film. In the heat fusion perforation method, a thermoplastic resin film is perforated by fusing the film by using a heated embossed roll.
In the above-described mechanical perforation methods, however, the size of each pore is as large as 100 .mu.m, and it is difficult to form smaller pores. In addition, the above-mentioned pores cannot be formed at a high density (e.g., 5,000 or more per 1 cm.sup.2).